The invention concerns intervertebral disc prostheses.
European Patent 0 356 112 discloses a prostheses comprising two plates and a body of compressible material interposed between and fixed to the plates. The prosthesis replaces the natural disc after the disc has been ablated, with the plates bearing against the vertebral plates of the adjacent vertebrae. With such prostheses it is possible to a great extent to reproduce the mechanical behavior of a healthy natural disc, in particular in compression or in torsion about any axis perpendicular to the longitudinal direction of the spine. However, the prostheses do not satisfactorily imitate two other movements: the relative rotation of the two plates about a main axis of the prosthesis or the axis of the spine, and the relative displacement of the two plates by shearing or sliding in a plane perpendicular to this axis. For these two movements, the known prostheses offer an insufficient mechanical reaction and are too rigid for the first movements.
It is therefore an object of the invention to more closely imitate the behavior of a healthy natural intervertebral disc.
The invention provides an invertebral disc prothesis comprising two plates and a deformable body interposed between the plates, where at least one of the plates comprises at least one contact stud which is able to stress the body in a direction not parallel to a main axis of the prosthesis and is movable relative to the body.
The invention also provides an intervertebral disc prosthesis comprising two plates and a deformable body interposed between the plates, where at least one of the plates comprises at least one contact stud which is movable in the body.
Thus, the contact stud offers mechanical resistance when the prosthesis is subjected to a rotational stress about its main axis or a shearing stress in a direction perpendicular to this axis. In addition, this resistance is variable depending on the relative position of the plates, for example depending on how close they are to each other and/or how inclined they are relative to each other. Indeed, the resistance to shearing and to rotation mentioned above will be greater the closer the contact stud is to the opposite plate and thus the more compressed the body is on the axis. Moreover, depending on the position of the contact stud, a relative inclination of the plates will bring the contact stud closer to the opposite plate and thus increase the resistance of the prosthesis locally near the contact stud. In contrast, the further away the contact stud is from the opposite plate the more the resistance is reduced. The prosthesis thus has a mechanical behavior which varies depending oh the relative position of the plates, which makes it similar to a healthy natural disc. If its dimensions are sufficiently great, the contact stud or each contact stud can additionally constitute an abutment limiting one of the relative movements of flexion or translation of the plates. Of course, what has just been described above for one contact stud is valid a fortiori when the prosthesis comprises a plurality of contact studs.
The contact stud is advantageously offcentered relative to the plate which bears it.
Thus, the resistance behavior of the prosthesis largely depends on the axis of the flexion and on the direction of this flexion.
The contact stud advantageously extends at a distance from the plate other than the one which bears it when the prosthesis is not stressed.
The contact stud advantageously has a length of between about 0.60 d and about 0.90 d, where d is a distance separating the two plates when the prosthesis is not stressed.
Advantageously, for the plate or each plate provided with at least one contact stud, the body is immobilized relative to the plate, in respect to a displacement parallel to the plate, only by virtue of the contact stud.
Thus, the body bears, without anchoring, on the plate or each plate comprising a contact stud. Assembly is thus carried out simply by stacking the plates and the body together. The body is, in particular, able to be separated from the plate under the effect of traction causing a displacement in the direction away from the plate, which is not conceivable in the normal conditions of use of the prosthesis.
The contact stud advantageously extends in a recess of the body opening into a lateral face of the body.
The contact stud advantageously has a flattened shape in a plane radial to the main axis of the prosthesis.
Thus, in the event of shearing or torsion about the main axis of the prosthesis, the bearing surface between the contact stud and the body is considerable, resulting in good distribution of the load, although the volume of the contact stud can be relatively small.
The contact stud advantageously has a flattened shape in a plane tangential to a direction circumferential to the main axis of the prosthesis.
The contact stud advantageously has a cylindrical lateral face, and the body has a cylindrical face bearing on the cylindrical face of the contact stud.
The contact stud advantageously has a lateral face extending outside the body.
Thus, the contact stud encroaches moderately on the volume of the body.
The lateral face of the contact stud advantageously extends in the continuation of an outer lateral face of the body.
Advantageously, for the contact stud or each contact stud, the plate other than the one bearing the contact stud has a recessed zone constituting the part facing the contact stud when the prosthesis is at test.
Thus, in the event of flexion or of the plates coming extremely close to each other, the contact stud or each contact stud does not come into abutment against the opposite plate, so that the deformable body taking up the stresses continues to impose the mechanical behavior of the prosthesis.
The plate advantageously comprises at least two contact studs arranged symmetrically about a center of the plate.
Each plate advantageously comprises at least one contact stud, the contact studs overlapping in a direction parallel to the main axis of the prosthesis when the prosthesis is not stressed.
The anti-shearing effect is increased in this way.
The contact studs advantageously overlap by a length of between about 0.35 d and about 0.65 d, where d is a distance separating the two plates when the prosthesis is not stressed.
Advantageously, when the prosthesis is not stressed, the contact studs overlap by a length of between about 0.45 h and about 0.85 h, where h is a height of the contact studs parallel to the main axis of the prosthesis.
Each plate advantageously comprises at least two contact studs, the contact studs being arranged alternatingly around the main axis of the prosthesis.
Other characteristics and advantages of the invention will also become apparent from the following description of two preferred embodiments given as nonlimiting examples.